This invention relates to hydrophobic association polymers and to their use in aqueous media for viscosity enhancement.
As taught in Handbook of Water-Soluble Gums and Resins, McGraw-Hill Book Publishing Co. (1980), it is known that the rheology of an aqueous medium can be modified by the addition of a water-soluble gum or resin hereinafter called water-soluble polymer. Such water-soluble polymers include polyacrylamide, acrylamide/acrylic acid copolymer, sodium polyacrylate, carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, polysaccharide as well as naturally occurring gums such as guar gum and chemically modified gums such as hydroxypropyl guar gum. As a result of this capability, there are many existing, as well as potential, industrial applications for controlling rheology of aqueous media with such water-soluble polymers. These applications include (1) drag reduction fluids to reduce energy required to pump aqueous fluids through pipes and other conduits, (2) thickening formulations to improve various properties such as liquid flow, handling and the like, and (3) formulations to control droplet size in agricultural sprays to reduce spray drift.
Unfortunately, however, the aforementioned conventional water-soluble polymers suffer from many serious deficiencies or limitations in actual use in such industrial applications. For example, for reasons of efficiency and economical considerations, it is common to employ very high molecular weight versions of such polymers. However, during many industrial applications, the conditions present during the practice of the application involve exposing an aqueous medium containing the high molecular weight water-soluble polymer to high shear. Such shear often causes mechanical degradation of the polymer and thus reduces the viscosity of the aqueous medium. While lower molecular weight polymers are less sensitive to shear degradation, they must be used in much higher concentrations in order to achieve the desired level of viscosity.
Secondly, while ionic water-soluble polymers such as neutralized acrylamide/acrylic acid copolymer, sodium polyacrylate, polystyrene sulfonate and the like are more efficient thickeners in deionized water than their nonionic counterparts, their thickening ability is greatly reduced by the presence of electrolytes such as sodium chloride, calcium chloride and magnesium sulfate in the aqueous medium. Such electrolytes are present in the aqueous media employed in most industrial applications, particularly those requiring the use of ground waters in subterranean formations as in enhanced oil recovery.
Finally, in many applications, the aqueous medium thickened with water-soluble polymer is exposed to temperatures in the range of 30.degree. C. to 100.degree. C. which normally causes reduction of viscosity. Such high temperatures are particularly common in enhanced oil recovery applications wherein the aqueous medium is pumped underground to depths of 5,000 to 20,000 feet, as is common for mobility control fluids and packing fluids.
In attempts to overcome some of the aforementioned deficiencies of the conventional water-soluble polymers, it has been a common practice to cross-link the polymer in order to improve resistances to thermal as well as shear degradation. See, for example, U.S. Pat. No. 3,247,171. Such attempts have generally not been successful. More recently, as taught in U.S. Pat. No. 3,984,333, an aqueous medium has been thickened by dissolving a water-soluble block polymer having water-soluble blocks and water-insoluble blocks in the aqueous medium. While such water-soluble block polymers apparently exhibit reasonably good resistance to shear degradation, such polymers are difficult and often impractical to prepare. More importantly, such polymers do not exhibit significant tolerance to electrolytes normally present in the aqueous media to be thickened.
While the cellulosic derivatives such as hydroxyethyl cellulose and biopolymers exhibit acceptable tolerance to the presence of electrolytes, cellulosic derivatives are generally ineffective at the low concentrations that are economical and exhibit poor thermal stability. The biopolymers such as xantham gums exhibit acceptable thermal stability, resistance to shear degradation and electrolytic tolerance. Unfortunately, such biopolymers are generally very expensive and are susceptible to biodegradation.
In view of the aforementioned deficiencies of conventional water-soluble polymers, it is highly desirable to provide a relatively inexpensive polymer which provides rheological control in aqueous media and exhibits thermal stability, electrolytic tolerance and good resistance to shear and biological degradation.